FIG. 1 is an exploded perspective view illustrating a conventional surface light source device (a backlight). A light guide plate 14 and a light source (not illustrated) are accommodated in a frame 12, a diffusion sheet 16 and prism sheets 17 and 18 are stacked on the light guide plate 14, and a cover 19 is placed thereon to integrally form a surface light source device 11. A lower surface of the light guide plate 14 is adhered to a bottom surface of the frame 12 by a double-sided adhesion tape 13, and the diffusion sheet 16 and the prism sheets 17 and 18 are adhered to an upper surface of the light guide plate 14 by a double-sided adhesion tape 15. The cover 19 is attached to the frame 12 such that an inwardly-projected claw 21 of the cover 19 is press-fitted or latched to the frame 12. A window 20 is opened in the cover 19 in order to expose an emission surface of the light guide plate 14.
As described above, in the surface light source device 11, the light guide plate 14 is fixed to the frame 12 by the double-sided adhesion tape 13, and the diffusion sheet 16 or the like is fixed to the light guide plate 14 by the double-sided adhesion tape 15. Therefore, a rattle sound (hereinafter, referred to as a rattle noise) caused by a slip of the surface light source device 11 is prevented, and a positional shift of each member is prevented.
However, in the structure in which the light guide plate and the like are fixed by the double-sided adhesion tape, the double-sided adhesion tape is expensive, and an effort of fixing the light guide plate and the diffusion sheet using the double-sided adhesion tape is also required, which results in a problem in that assembly cost of the surface light source device increases. When the light guide plate and the like are fixed only by the double-sided adhesion tape, the double-sided adhesion tape may be peeled off due to vibration or heat. When the double-sided adhesion tape is partially peeled off, the slip of the light guide plate is generated in a length direction (8 direction in FIG. 1) of the double-sided adhesion tape, and the chain-reaction peel-off is generated in the double-sided adhesion tape by the slip, which results in the generation of the rattle noise. When the slip of the light guide plate or the like is generated and the double-sided adhesion tape is peeled off, for example, a gap is generated between an edge of the light guide plate and an edge of a window of a bezel cover, and a bright line is generated due to a screen tear. Similarly, a collective property may be degraded due to the shift of the prism sheet. Since the double-sided adhesion tape is sandwiched between the diffusion sheet and the light guide plate, a thickness of the light guide plate disadvantageously increases.
For example, in a surface light source device disclosed in Japanese Unexamined Patent Publication No. 2003-207767, outer peripheries of a plurality of rectangular optical sheets are bonded by a bonding agent, and the outer periphery of a gap between the sheets is sealed. In this case, the plurality of sheets are integrally bonded to each other in one side on an opposite side of a light source by a non-plastic bonding agent layer in which a shear deformation is not generated. The plurality of sheets are integrally bonded to each other in other three sides by a plastic bonding agent layer in which the shear deformation can be generated by a stress caused by a difference in thermal expansion. Similarly, a plurality of integrated optical sheets and the light guide plate are integrally bonded to each other in one side on the opposite side of the light source by the non-plastic bonding agent layer in which the shear deformation is not generated, and the optical sheets and the light guide plate are integrally bonded to each other in other three sides by the plastic bonding agent layer in which the shear deformation can be generated by the stress caused by the difference in thermal expansion.
However, in the surface light source device having the above structure, the outer peripheries of the plurality of optical sheets are bonded, and the outer peripheries of the optical sheet and the light guide plate are also bonded. Therefore, the cost of the bonding agent increases, and the thickness of the surface light source device increases. Since a bonding margin is required in the outer peripheries of the optical sheet and the light guide plate, a region that can be used as the emission surface is narrowed in the light guide plate to degrade use efficiency of the light guide plate. In the case where a foreign substance invades between the optical sheets or between the optical sheet and the light guide plate in a surface light source device assembling process, there is a problem in that the foreign substance cannot be removed.